Although the impact resistance of polycarbonate resins, and especially aromatic polycarbonate resins, is generally thought to be good, the sensitivity of such resins to thickness in Izod impact tests is well known. For instance, notched Izod values for a one-eighth inch thick section of molded polycarbonate can be in excess of 16 ft.lbs/in., but only several ft.lbs/ in. for a one-quarter inch section.
It has been proposed that this thickness sensitivity can be reduced by adding a small amount of a polyolefin, e.g., polyethylene, to the polycarbonate resin. While the addition of polyethylene has proven to be somewhat effective, in the main thick section notched Izod strengths are still not comparable to thin section strengths.
Serini, et al, in U.S. Pat. No. 4,172,103, discloses thermoplastic molding compositions of an aromatic polycarbonate, a thermoplastic resin and a modified or unmodified rubber. The polycarbonate resin contains at least 50 percent of tetraalkyl substituted structural units; the thermoplastic resin can be, for example, polystyrene; and the rubber can be, for example, ethylene-propylene-diene rubber, among others. These compositions are described as being very homogeneous and as characterized by a number of improved properties, including high stability to saponification, good notched and unnotched impact strength and high stress cracking resistance.